The invention relates to a high critical temperature (HTc) superconductive multifilament strand and to a method of making such a strand. More particularly, the invention relates to an HTc superconductive multifilament strand clad in silver and used with AC, and to a method of making such a strand.
The use of HTc superconductive multifilament strands with AC requires good decoupling of the HTc superconductive filaments making it up in order to limit energy losses due to induced currents.
It is known to make HTc multifilament strands by the xe2x80x9cpowder in tubexe2x80x9d technique. That consists in filling a billet with powder reagents that are suitable, after heat treatment, for transforming into a superconductive material, and in particular into a material of the HTc ceramic type.
The billet is then closed under a vacuum and drawn down, after which it is put into a bundle in a new billet itself in turn closed under a vacuum and then drawn down. The resulting multifilament strand may be subjected to the same steps, and so on until a desired number of filaments per unit area has been obtained.
The strand made in this way is then put into its final form, e.g. by rolling and/or twisting, and is then subjected to heat treatment to transform its powder reagents.
The material constituting the billets must be sufficiently ductile to be capable of withstanding the various drawing-down and rolling stages, and its composition must be inert or at least without consequence for the heat treatment that transforms the powder reagents into a superconductive phase. It is known that silver can be used as the material constituting the billets.
However, silver is a material that is very highly conductive at the operating temperatures of HTc superconductors. As a result there is practically no electrical decoupling between the filaments.
It is known that Ag can be doped with impurities of the Pd or Au type to 1% or 2%. That technique makes it possible to gain two decades in resistivity at 20 K.
That technique can also be used for applications at 77 K. However, the Ag/Pd alloy is expensive which makes it economically inconceivable in mass production applications.
In addition to increasing the resistivity of the silver-based matrix, it is also known to twist the conductor at a very small pitch with a very small filament diameter. However the resulting decoupling is not sufficient in most AC applications.
One of the objects of the present invention is to propose a multifilament strand in which filament decoupling is significantly improved.
To this end, the invention provides a powder in tube type method of making an HTc superconductive multifilament strand having a silver-based matrix, in which:
in a monofilament step, a first silver-based envelope is filled with powder reagents suitable, after heat treatment, for transforming into an HTc superconductive material;
the resulting billet is drawn down into a monofilament strand with a cross-section that is square or rectangular in shape;
in a first multifilament step said monofilament strand is cut into lengths and a secondary silver-based envelope of square or rectangular section is filled with the resulting lengths, thereby making a multifilament billet, the multifilament billet being drawn down in turn to form a multifilament strand of square or rectangular section;
in a secondary multifilament step, that is performed at least once, said multifilament strand is cut up into lengths and a new silver-based envelope of square or rectangular section is filled with the resulting lengths, thereby making a new multifilament billet, the new multifilament billet being drawn down in turn to form a new multifilament strand of square or rectangular section;
the new multifilament strand is shaped; and
heat treatment is applied to the shaped strand;
according to the invention, at least one face of the monofilament strand is electrically insulated; and
during the first multifilament step the silver-based secondary envelope of square or rectangular section is filled with the resulting lengths, thereby making the multifilament billet.
In one implementation, a layer of electrically insulating material is deposited on the faces to be insulated of the lengths of the monofilament strand.
In another implementation, while making the multifilament billet of the first multifilament step, an electrically insulating material is interposed between the faces to be insulated of the lengths of monofilament strand placed in the envelope.
According to another characteristic of the method of the invention, during a secondary multifilament step at least one of the faces of the multifilament strand is insulated; then
the new silver-based envelope of square or rectangular section is filled with the resulting lengths, thereby making a new multifilament billet.
In an implementation, a layer of electrically insulating material is deposited on the faces to be insulated of the lengths of the multifilament strand.
In another implementation, while making the multifilament billet of the secondary multifilament step, an electrically insulating material is interposed between the faces to be insulated of the lengths of multifilament strand placed in the envelope.
The invention also provides an HTc superconductive multifilament strand having a cross-section that is generally square or rectangular in shape, comprising a plurality of superconductive filaments, each superconductive filament having a cross-section that is generally square or rectangular in shape, comprising a core of HTc superconductive ceramic, said HTc superconductive ceramic core being surrounded by Ag cladding.
According to the invention, each superconductive filament includes an electrically insulating layer on at least one of the faces of its Ag cladding.
A first advantage of the present invention results from better decoupling characteristics of the filaments because of the existence of an insulating layer on at least one face of each filament.
Another advantage of the present invention results from a reduction in the cost of fabricating a strand of the invention.
Another advantage of the present invention results from the fact that the method proposed by the invention can be implemented on existing fabrication devices.